1. Field of the Invention
This invention relates to cordless telephones. More particularly, the invention concerns an apparatus and method for accelerating the establishment of a communication link between the handset and the base of a cordless telephone system.
2. Background Discussion
Cordless telephones normally consist of two separate stations, a base station and a handset station. The base station is plugged into a telephone outlet and comprises all the necessary equipment and electronics for interfacing with the local telephone trunk. The handset station comprises all the electronics and equipment necessary for interfacing with the user of the telephone. Such equipment includes a touch-tone key pad for dialing telephone calls and providing other features, as well as a speaker and microphone. Both stations also include radio transceivers and antennae for radio communication between the handset and the base station.
Since the base station is essentially stationary and usually must be hardwired to a telephone outlet, it will typically receive its electrical power supply from an AC wall outlet. The telephone handset, however is intended to be portable and cannot be designed for connection to an AC power outlet. Therefore, the handset receives its power from an on-board battery pack. Typically, the batteries are of the rechargeable type, and are recharged when the handset is placed in an accepting cradle on the base station and electrically coupled to the base station power supply via a recharge circuit.
In addition to the analog form of conversation that takes place on the telephone, digital signals for governing protocols between the two separate stations are also transmitted and exchanged. Typically, protocol control is carried out by transmitting digital command words and digital acknowledgment words (digital data words, collectively) between the two stations. This may be accomplished on a separate frequency or channel than the voice communications or may be accomplished directly over the voice frequency channels.
When transmitting digital words through the air via radio signals, there is a possibility that the transmitted data may not be received correctly at the receiving station. Therefore, error detection techniques are used in cordless telephones. A particular form of error detection is described in U.S. Pat. No. 4,982,401, entitled "Method and Apparatus for Detecting Transmission Errors in Cordless Telephones", issued Jan. 1, 1991, and assigned to the assignee of the present invention.
In order for a user to initiate a call on the cordless telephone handset, a protocol must be exchanged between the handset and the base station to initiate a call placed on the cordless telephone through the base station unit to the telephone lines. As well, when an incoming call is received by the base unit, communication to the cordless handset, which may be located remote from the base unit, must also be initiated also using a protocol exchange.
Each telephone set, comprised of the base station and the cordless handset station, is programmed with a security code. The security code prevents two telephone systems which may be located in the transmitting vicinity from causing crosstalk between the two separate systems. Thus, a transmission from the handset or base of one system will not be accepted by the other system because it will not recognize the security code. If there is a match of security codes, this indicates that the handset and the base station are on the same system.
Establishment of the communication protocol between the handset and the base unit of a cordless telephone occurs in a packet form, comprised of originating and answering packets. Of course, either the handset or the base can transmit the originating packet depending upon whether a call is placed from the cordless handset out or a call is an incoming call. If, for example, a handset unit is the initiating unit, it initiates a conversation by transmitting an originate packet to the base unit.
In prior art cordless telephones, the originate packet is comprised of the following parts. First, a header portion, which is a pattern sent at the beginning of the packet. The purpose of the header in the originate packet is to indicate to the receiving unit, which may be the base unit, that a call is being placed from the cordless handset and that the information it is receiving is a packet or protocol command rather than an audio transmission.
The packet or protocol command may be an exchange of packets to establish communication between the cordless and base units or may be the transmission of digital signals representing, for example, the telephone number of the call being placed by the user. Of course, the form of the packets for initiating a call and for transmitting a telephone number called will be different.
In prior art devices, the header portion of the packet is followed by a security code. The security code, as mentioned earlier, is used to match handsets and base units on the same system. Only a unit with a matching security code, whether that unit be the handset or the base station, will respond to the packet. In addition to security code, other items such as data commands, packet format identifiers, packet sequence numbers, check sums, and the like, may be included in the originate packet. The answer packet is similar to the originate packet as it includes a header, a security code, and other items. In prior art devices, an initiating unit, which again may be either the handset or the base station, will transmit an originate packet to the other unit. Once the entire originate packet has been received and the receiving unit has determined the packet was properly sent to it by identification and matching of the security code, the receiving unit will then transmit an answer packet to the initiating unit to inform the initiating unit that the packet was received correctly and that transmission of data can then proceed. If the originating unit does not receive the answer packet within a certain period of time, it will retransmit the originate packet and wait once again for an answer packet from the receiving unit. This may occur several times as designed by the programmer until the originating unit either receives a correct answer packet, or determines, by the passage at a predetermined time, for example, that transmission cannot be accomplished. The originating unit then stops transmitting the originate packet and an indication of such event given to the user.
A problem attendant with the prior art method of sending originating and waiting for responding packets, is that there is often a time lag between the time the user may attempt to place a call and the time that the user receives a dialtone. In many prior art cordless telephones, taking the telephone off-hook, does not cause the generation of a dialtone. This is because the cordless telephone is meant to be portable and carried about by the user. To automatically generate a dialtone, as is a common usage with hand wired, non-cordless telephone systems, when the handset is lifted would obviously not prove to be useful. Instead, most cordless telephones are equipped with an initiation or talk switch, which when pressed, will cause a series of events to occur and eventually a dialtone to be generated. Due to the time lag inherent in the prior art seriatim originate and answer packet, there may be a delay of some time between the time the user presses the button to initiate a call and the receipt of a dialtone. In the reverse situation, when a call is received by the base unit and is transmitted to the cordless phone, once again the user will have to press talk or other buttons and, due to the time lag inherent in the originate and receiving answering packet, there will be a time lag between the time the telephone rings and the time the user may begin conversation with the caller.
After the user of the cordless handset has received a dialtone indicating communication with the base station, numbers are pressed corresponding to the telephone number to be called. This will cause a lag, in prior art herein, due to the delay between the originate and answer packets acknowledging receipt of a signal corresponding to, for example, the number 4. The delay inherent in the prior art systems reduces the speed with which the user can initiate a call as compared to the now popular push button tone system which facilitates the rapid receipt of a dialtone and rapid initiation of a telephone call after the dialtone has been received.
In addition, in some prior art devices, the security code bits and the data bits sent by an initiating unit were exclusive--ored (XOR) or otherwise interleaved. The receiving unit would then be required to perform a similar function and separate out the data from the security code. The result was that there was an inherent time delay in determining whether the security codes of the initiating and receiving units matched. As well, in such prior art devices, the overall length of an originate packet, whether to initiate communication or to send key pad selections, would be of the same length due to the interleaving of the security code and the data to be sent.
Therefore, it is an object of the present invention to provide an improved cordless telephone set.
It is another object of the present invention to provide a cordless telephone set in which the initiation of outgoing calls and the receipt of incoming calls to the cordless handset is improved.